In this century, the printed electronics industry has developed as a nanotechnology-based environmentally friendly convergence industry and has been considered a new paradigm to overcome the limitations of existing industries. In the printed electronics industry, new concepts of electronic materials and components are produced based on printing processes for mass production at low temperature and ambient pressure, achieving low cost, flexibility, and large area of products.
Under such circumstances, it is anticipated that a new market for electronic products will be created in the future in response to emotions, consumption patterns, and diverse needs of consumers and its size will surpass that of the existing markets. Numerous printed electronic products have been developed, for example, RFIDs, memories, a variety of displays (for example, OLEDs, ELs, electronic papers, and flexible displays), lighting devices, batteries (for example, secondary batteries and solar cells), touch panels, sensors, organic transistors, printed circuit boards (for example, PCBs and FPCBs), films for electromagnetic interference (EMI) shielding and transparent electrodes (including metal mesh types), and other applied products in various fields. These printed electronic products have opened up new markets. With the emergence of price competitive printed electronic products whose devices are freely designable, their market is expected to expand. Conventional processes for device production are partially limited by the kind and size of substrates employed, but printing processes are applicable irrespective of the kind, shape, and size of substrates. Particularly, printing processes are easily applied to large-size or flexible substrates and are recognized to be innovative in mass production of single products as well as small quantity batch production.
Suitable inks are essential for the manufacture of printed electronic products. Particularly, conductive inks are considered the most important materials for a variety of electrodes (including transparent electrodes). Specifically, a conductive ink composed of conductive materials, for example, a metal, an alloy, a metal oxide, carbon nanotubes (CNTs), graphene, graphite, a conductive carbon, a conductive polymer, conductive nanoparticles or nanowires, or a precursor thereof, is directly printed (or coated) with an inkjet printer or a suitable printing system, such as a gravure printing, flexo printing, (rotary) screen printing, offset printing, gravure-offset printing or (nano)imprinting system, followed by drying or sintering to form a metal wire with a desired shape. This is essential for printed electronics processes.
Conductive inks necessary for printed electronics processes have been investigated and developed by many researchers. Generally, nanoparticle-based inks suffer from poor long-term storage stability or undergo aggregation of particles or precipitation, causing nozzle clogging during printing. For the purpose of preventing such problems, polymeric materials are usually used as stabilizers. However, excessive use of the stabilizers increases the viscosity of the inks or causes other problems, such as increased surface tension, high sintering temperature, and increased conductivity.
Conductive inks using metal nanoparticles can be found, for example, in Nanotechnology, 17, p 2424 (2006), J. Mater. Res., 24, p 1828(2009), J. Colloid Interface. Sci., 273, p 165(2004), J. Mater. Chem., 19, p 3057(2009), US 2010/0084599 A1, US 2010/0009153A1, and US 2011/0183128A1.
The most commonly used approach to solve the problems of metal inks in the form of nanoparticles is, for example, to use metal precursors, including organometallic salts and complexes. However, silver-containing carboxylic acid salts are generally sensitive to light, are not readily soluble, and have a high decomposition temperature, which limit their applicability despite ease of production. Attempts to solve such problems have been made, for example, by the use of silver precursors in which an electron donor, such as an amine or phosphine compound, is coordinated to a fluorinated carboxylic acid or a silver carboxylate having a long alkyl chain (Chem. Vapor Deposition, 7, p 111 (2001), Organometallics, 15, p 2575 (1996), Chem. Mater., 16, p 2021 (2004), and J. Chem. Crystallography, 26, p 99 (1996)). Such inks are described in the literature: for example, inks using such an organometallic complex or metal salt (U.S. Pat. No. 7,691,294 B2, US 2011/0111138A1, U.S. Pat. No. 8,226,755 B2, and J. Am. Chem. Soc., 134, 1419, 2012), an ink containing silver β-ketocarboxylate (WO 2007/004437A1), and inks using a silver neoalkanoate (Makromol Rapid Commun, 26, p 315 (2005), J. Mater. Sci., 41, p 4153 (2006), Chem. Mater., 21, p 343 (2009) and US 2011/0008548A1)). In addition to the silver precursor inks, copper and aluminum precursor inks using other ink materials are currently being developed (Organometallics, 20, p 4001 (2001), US 2008/0003364A1, Adv. Mater., 23, 5524, 2011, WO 2009/059273A2, and WO 2010/011974A1).
However, such conductive inks are insufficient in conductivity or have poor adhesion to substrates upon formation of metal wires in the form of thin films by sintering, limiting their application to various products where high reliability is needed.
The present inventors have made continued efforts to solve the problems of the prior art and finally arrived at the present invention.